In Situ PM-IRRAS Studies of an Archaea Analogue Thiolipid Assembled on a Au(111) Electrode Surface

Abstract

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) has been applied to determine the conformation, orientation, and hydration of a monolayer of 2,3-di-O-phytanyl-sn-glycerol-1-tetraethylene glycol-dl-alpha-lipoic acid ester (DPTL) self-assembled at a gold electrode surface. This Archaea analogue thiolipid has been recently employed to build tethered lipid bilayers. By synthesizing DPT(d16)L, a DPTL molecule with a deuterium substituted tetraethylene glycol spacer, it was possible to differentiate the C-H stretch vibrations of the phytanyl chains from the tetraethylene glycol spacer and acquire the characteristic IR spectra for the chains, spacer, and lipoic acid headgroup separately. Our results show that the structure of the monolayer displays remarkable stability in a broad range of electrode potentials and that the phytanyl chains remain in a liquid crystalline state. The tetraethylene glycol chains are coiled, and the IR spectrum for this region shows that it is in the disordered state. The most significant result of this study is the information that in contrast to expectations the spacer region is poorly hydrated. Our results have implications for the design of a tethered lipid membrane based on this thiolipid.

Effect of the Structure of Cholesterol‐Based Tethered Bilayer Lipid Membranes on Ionophore Activity

“…As pure EO 3 C SAMs show no changes in resistance or capacitance upon addition of ionophores, we propose that they are incapable of solvating ions due to the ethylenoxy linker layer being largely dehydrated. This is supported by independent infrared reflection absorption spectroscopy studies by Leitch et al [15] and McGillivray et al [14] on similar tethering molecules. However, we expect the degree of dehydration found will be lower due to the smaller cross-sectional area of cholesterol (32 2 ) [41] versus these lipids (69 2 for DPhyPC).…”

“…[10][11][12] Recently, independent studies by Losche et al and Lipkowski et al showed that pure monolayers of the thiolipids 20-tetradecyloxy-3,6,9,12,15,18,22-heptaoxahexatricontane-1-thiol (WC14) and 2,3-di-O-phytanyl-sn-glycerol-1-tetraethylene glycol-dl-a-lipoic acid ester (DPhyTL), respectively, exclude water from the ethylenoxy layer in a tBLM system. [14,15] A similar situation is likely to apply to EO 3 C used in this study. Hydration can be introduced by mixing diluting thiols, in our case 6-mercaptohexanol (6MH), into the tethering layer.…”

4 Electrochemistry of Biomimetic Membranes

“…However, at -0.600 V vs. Ag/AgCl(0.1 M KCl) a pronounced increase in the neutron scattering length density of the spacer was observed, denoting an increased amount of water transferred into this region. Leitch et al 168 drew similar conclusions using polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). Thus, the fraction of nonhydrated C=O of the lipoic acid ester group was found to be about 50% at the more positive potentials and to attain a value of about 30% at -0.600 V, which denotes an increasing hydration of the spacer at these negative potentials.…”

“…The pzc equals -0.435 V/SCE for Hg 188 and -0.040 V/SCE for polycrystalline Au. 189 Therefore, an appreciable hydration of the TEO moiety, possibly accompanied by its elongation 3 , takes place in the proximity of a rational potential of about zero on Hg, but at a much more negative rational potential of about -0.600 V on Au, 167,168 close to the DPTL desorption from this metal. A drawback in the use of mercury-supported tBLMs is represented by the notable difficulty in using surface sensitive techniques for their structural characterization.…”

Planar Block Copolymer Membranes by Vesicle Spreading

“…It was reported that lipid bilayers covalently bound to gold via LA anchors exhibit a slight tilt relative to the surface. [34,35] Furthermore, a tilt was also observed in the case of surface-grafted triblock copolymer membranes. [13] The covalent attachment of PB-PEO-LA films to gold probably also led to tilted films, thus to decreased thicknesses.…”